116
N . W. MOORE
in the growth and multiplication of protozoa. The surviving mosquitoes
fared better in the treated samples than in the controls because more
food was available for them a t an early stage of development.
VII. PICSTICIDES A N D EVOLUTION
Pesticides exert strong selective pressure 011 pest populations. It is
not surprising that resistant strains of pests are quickly selected; but
the range of preadaptations to pesticides present in pests has been
remarkable. By 1940 resistance to pesticides had been recorded in eight
insect species (Georghiou, 1965a); by 1961 at least 137 speciesofpesthad
become resistant to pesticides. For a review of the earlier literature see
Brown (1968). Two main types of resistance have been recorded. In
some species the resistant strain has a biochemical or physiological
property which prevents pesticides damaging the insect, for example a
detoxicating enzyme. In others the insect has a behavioural property
which reduces contact with lethal doses of the insecticide. Many pests
become resistant to several pesticides as a result of exposure to them
(multiple resistance) but in addition many cases of cross-resistance are
known: in these resistance acquired to one pesticide confers resistance
to one or more other pesticides. Frequently cross-resistance is between
related chemicals, for example between those of the cyclodiene-derived
group, but cross-resistance is also known between organochlorine and
organophosphorus and carbamate insecticides. Distinct patterns of
cross-resistant types are emerging (Winteringham and Hewlett, 1964).
The generation time of arthropods is so much shorter than that of
vertebrates that it is not surprising that resistance has been most
frequently observed in this group. However, in recent years it has been
reported in fish (Vinson et aE., 1963) and mammals (Boyle, 1960) and
possibly in amphibia (Boyd et al., 1963). Strains which are 1.7
times as resistant to DOT as the controls, have been produced experimentally in mice in nine generations (Ozburn and Morrison, 1962).
Cross-resistance has been demonstrated in fish (Boyd and Ferguson,
1964).
The ecologically relevant effects of pesticide resistance are these:
1. It causes a change in spraying programmes and hence in the
nature of the pesticide factor. The change may be towards higher
doses or to new chemicals.
2. I n a sprayed area vertebrate species are unlikely to develop resistance before invertebrate pests.
3. If a species achieves resistance by being able to store more pesticides it would become a more effective biological concentrator of
pesticides, and hence a greater hazard to its predators.
4. Genes conferring resistance may be linked with those conferring
N . W. MOORE
in the growth and multiplication of protozoa. The surviving mosquitoes
fared better in the treated samples than in the controls because more
food was available for them a t an early stage of development.
VII. PICSTICIDES A N D EVOLUTION
Pesticides exert strong selective pressure 011 pest populations. It is
not surprising that resistant strains of pests are quickly selected; but
the range of preadaptations to pesticides present in pests has been
remarkable. By 1940 resistance to pesticides had been recorded in eight
insect species (Georghiou, 1965a); by 1961 at least 137 speciesofpesthad
become resistant to pesticides. For a review of the earlier literature see
Brown (1968). Two main types of resistance have been recorded. In
some species the resistant strain has a biochemical or physiological
property which prevents pesticides damaging the insect, for example a
detoxicating enzyme. In others the insect has a behavioural property
which reduces contact with lethal doses of the insecticide. Many pests
become resistant to several pesticides as a result of exposure to them
(multiple resistance) but in addition many cases of cross-resistance are
known: in these resistance acquired to one pesticide confers resistance
to one or more other pesticides. Frequently cross-resistance is between
related chemicals, for example between those of the cyclodiene-derived
group, but cross-resistance is also known between organochlorine and
organophosphorus and carbamate insecticides. Distinct patterns of
cross-resistant types are emerging (Winteringham and Hewlett, 1964).
The generation time of arthropods is so much shorter than that of
vertebrates that it is not surprising that resistance has been most
frequently observed in this group. However, in recent years it has been
reported in fish (Vinson et aE., 1963) and mammals (Boyle, 1960) and
possibly in amphibia (Boyd et al., 1963). Strains which are 1.7
times as resistant to DOT as the controls, have been produced experimentally in mice in nine generations (Ozburn and Morrison, 1962).
Cross-resistance has been demonstrated in fish (Boyd and Ferguson,
1964).
The ecologically relevant effects of pesticide resistance are these:
1. It causes a change in spraying programmes and hence in the
nature of the pesticide factor. The change may be towards higher
doses or to new chemicals.
2. I n a sprayed area vertebrate species are unlikely to develop resistance before invertebrate pests.
3. If a species achieves resistance by being able to store more pesticides it would become a more effective biological concentrator of
pesticides, and hence a greater hazard to its predators.
4. Genes conferring resistance may be linked with those conferring